Water Activity 102: Microbial Growth

Food manufacturers must consider microbial growth when producing and packaging products. This webinar discusses factors that affect microbial growth, how water activity controls microbial growth, government compliance, common food pathogens, formulating a specific water activity, and hurdle technology.

Microbial failures in food packaging can lead to product recalls, costing a company millions of dollars in operational delays, legal fees, medical claims, food retrieval, food disposal, and medical care. They can also cause additional losses in reputational damage, loss of future sales, and loss of consumer confidence.

FAT TOM: Six factors impacting microbial growth

Six main factors affect microbial growth: food, acid, time, temperature, oxygen, and moisture. (A silly but effective pneumonic device to use is FAT TOM.) The nutritional composition of a food determines what microbes could grow. Acids, or pH levels, also affect microbial growth. Molds can grow at the lowest pH, while bacteria thrive in the highest pH levels and cannot grow in environments with a pH below 4.6.

Microbial growth is exponential, so it’s critical to catch dangerous pathogens early. We can stop pathogen growth by depriving the microbes of food and oxygen and introducing competing species. Different pathogens thrive at different temperatures. The food safety danger zone is between 4 and 60 degrees Celsius (40 to 140 degrees Fahrenheit). We can deactivate bacteria using a combination of elevated time and temperature treatments.

Consider microbial oxygen or redox potential. Aerobic microbes (like mold) require oxygen, while anaerobic pathogens (Clostridium botulinum and Bacillus cereus) do not need oxygen to grow. Some bacteria (like E.coli and Staph aureus) are facultatively aerobic, meaning that they can switch between aerobic and anaerobic depending on the environment. On the other hand, microaerophiles require oxygen in smaller amounts to grow (Campylobacter jejuni).

Moisture matters in pathogen growth through the food's water activity. Pathogenic bacteria, for example, grow only in water activities above 0.85, whereas spoilage with molds and yeasts has a water activity limit of 0.7. Conversely, no microbial growth can happen with a water activity lower than 0.6.

How water activity affects a microorganism

A difference in water activity means a difference in an energy level. With a high water activity, the water in the microbe wants to leave, which changes the trigger pressure within the microorganism. The microorganism will try to equal its energies with the water activity in the environment.

The microorganism will alter its membrane to reduce the water activity, thereby maintaining its trigger pressure. It might produce or transport amino acids or sugars in small quantities to try and reduce water activity. However, the water activity still won't match the environment, and the microorganism goes into stasis, or dormancy. 

Common food pathogens

Food pathogens fall into two categories: foodborne intoxication and foodborne infection. A foodborne intoxication is when one ingests a toxin produced in the food and becomes sick. A foodborne infection is when the toxin grows in the GI tract. (Intoxication forms in the food, and infection forms in the gut.)

Staph aureus can grow with or without oxygen, has the lowest water activity limit, and is easily cross-contaminated. Therefore, finding Staph aureus on food processing equipment is a sign of poor sanitation. This pathogen growth can be easily prevented using cleaning, sanitizing, appropriate preparation, and minimizing cross-contamination.

Botulism is anaerobic and won't grow at pH levels lower than 4.6. Just three minutes of boiling can kill botulism. It has a higher water activity limit. Botulism exists in nature—soil, plants, water—and in improperly canned foods and low-acid foods—beets, green beans, baked potatoes wrapped in foil, smoked fish, herb-infused oil, and honey.

Salmonella spp. is the number-one reported pathogen. This microbe is a foodborne infection and is more common in the summer months. Salmonella can thrive in oxygen-rich or oxygen-depleted environments, can be killed by cooking, and is found in contaminated feces, drinking water, person-to-person contact, egg products, raw fruits and veggies, unpasteurized milk products, and even in products like flour and peanut butter.

Listeria can grow in refrigerated temperatures, can be killed by cooking and pasteurization, and has a water activity limit of 0.92. Listeria exists in uncooked meats, vegetables, unpasteurized milk, and some soft cheese. In low-temperature, low-oxygen environments, Listeria can grow if present.

Most E. coli strains are harmless and are essential to the intestinal tract. The pathogen strain can be killed through cooking or pasteurization. E. coli has a low infectious dose and is difficult to kill. 

Bacillus cereus is anaerobic and has a short incubation time. This pathogen is often confused with the stomach flu.

Reducing water activity to prevent microbial growth

Reducing water activity will change what microbes can grow. Dehydrating a product can preserve the food and lower the water activity. Adding a humectant, either singularly or in groups, can also lower water activity. Hurdle technology involves introducing barriers to microbial growth, like managing temperature, water activity, acidity, and oxygenation.